An American Airlines flight from New York to New Delhi was diverted to Rome on Sunday afternoon following an "alleged bomb scare", an Italian airport spokesman said.

The US-based carrier gave no details on the nature of what it called a "possible security concern" on the flight carrying 199 passengers plus crew, which was escorted by two Italian fighter jets before landing.

"The flight landed safely at FCO (Rome), and law enforcement inspected and cleared the aircraft to re-depart," American Airlines said in a statement.

Mahesh Kumar, an IT consultant aboard the flight, said the pilot announced the diversion to Rome due to "security reasons" about three hours before landing. 

"Everyone was afraid. Everyone was staying quiet and obeying the orders," the 55-year-old from Texas told AFP.

"They asked us to sit down and not to roam around while the fighter jets were near us," Kumar said, adding that Italian police escorted passengers for a security screening in the airport when they landed.

The flight had taken off from New York's John F. Kennedy International Airport on Saturday at around 8:11 p.m. local time (01:11 GMT Sunday), according to flight tracker FlightAware.

It turned around sharply while flying over the Caspian Sea, the website showed.

An American Airlines official said the aircraft landed in Rome due to Indian protocol requirements.

"The possible issue was determined to be non-credible, but per DEL Airport protocol, an inspection was required before landing at DEL," the official said, speaking on condition of anonymity. 

"The flight will stay in FCO overnight to allow for required crew rest before continuing to DEL as soon as possible tomorrow."

Rome airport operations were not affected by the incident, an Italian airport spokesman said.

The US Federal Aviation Administration (FAA) confirmed the plane had landed safely "after the crew reported a security issue."

American Airlines, one of the largest US air carriers, is headquartered in Fort Worth, Texas.

Its international operations serve more than 60 countries, according to the airline's website.